Permanent magnet assembly for electrodynamic loud-speakers



F. J. HARBAUGH Q PERMANENT MA ET ASSEMBLY FOR ELECTR AKERS March 21, 1950 ODYN C LOUDSPE iled Nov. 50, 194

Fred l Harbcigh A'rronNsY INVENTOR.

Lrhnt Y ,central bore functioning as part of the horn.

Patented Mar. 21, 1950 PERMAYEN :MAGNET All@ SEMBLY FOR ELECTRQYNAMIC LQ@ SPEAKERS Fred '.I. Harbaugh, lladdonield, N. assigner to Radio Corporation of America, a corporation of `Delaware Application November Sil, 1945, Serial No. 632,065 4 Claims. (Cl. 179-ll5.5)

1 This invention relates to sound transducers and more particularly to a permanent magnet dynamic loudspeaker.

Anobject of the invention is to provide a new opening t* in the bottom of the cup. A soft iron or mild steel inner pole piece Il is located upon the pole face or top surface of magnet l and the upper end of this pole piece cooperates with a and improved method and .means of assembly of 5 circular opening in yoke plate 3 in forming an magnetic elements of a loudspeaker. annular air gap.

More particularly it isan object of the inven- For the purpose of accurately centering the tion to provide improved and simplified means pole piece ll with respect to the opening i3 a 'for holding the elements of a permanent magsheet metal centering disc l5, preferably of spring net loudspeaker together in desired operating bronze or brass, is permanently secured to the relation. yoke plate t as by rivets lll or welding and is Still other objects and advantages of the in provided with a circular opening ccncentrically vention will be apparent from the :following specidisposed with respect to opening it in the yoke `fication. The features of novelty whichbelieve plate. This centering disc is formed with a to be characteristic of the invention are set iorth downward bias tendency as indicated by the dotwith particularity in the appended claims. The ted line lil. To improve the spring action the invention itself, however, both as to its fundacentral portion of the disc is preferably divided mental principles and as to its particular embodiinto independently acting segments, in the man ments will best be understood by reference to the ner disclosed in connection with Figure au. In specification and accompanying drawings in the position shown, the spring tension exerted which: downwardly by the disc l5 forces the pole piece Figure l is a cross sectional View in side eleil downwardly against the magnet l and mainvatios of a loudspeaker magnet structure, etc. tains goed magnetic Contact and holds the mas'- embodying the invention, netic elements il, 1 and l together in assembled Figures 2 and 3 are side elevation fragmentary g5 relation. Ordinarily, friction. resulting from the views, partly in section, of a loudspeaker magpressure exerted by disc l5 is suicient to prevent net system embodying modied forms of 'the displacement of magnet l laterally relative to invention, elements l and Il, but to insure against undue Figure l is a side elevation View in section of CSlDlaCSlll-ent Caused by dropping the lllt, limta loudspeaker magnet system embodying a modiing means ShOWn in Figures 4, 5 and 6 may be lied form of the invention, used. The periphery of the base of the magnet Figure 4a, is a plan view, reduced in size, of an l may be welded to the yoke l if desired. element shown in Figure 4, and The unit may be assembled as follows: The Figures 5 and Gare fragmentary views partly magnet l can be located in the position shown in section, reduced in size, of modifications emand the pole piece Il placed on top thereof as bodying the invention. indicated. Next, the yoke plate s can be dropped In the various figures like reference numerals into approximately the position shown and represent like parts. clamping pressure applied to force it downward Referring now more particularly to Figure 1, against the pressure exerted by the disc i5 onto the invention is illustrated by means of a speaker the ends of the cup-like yoke member l into the unit adapted foruse with a horn of the exponenposition indicated, the screws li being then tighttial type, not shown. A paramagnetic field cned to maintain the assembled position. Weldstructure comprising a cup-shaped field yoke I ing may be used to hold these members together, of ferromagnetic material such as cold rolled particularly in case a U-shape open type yoke steel or other parainagneticvmaterial is provided member used, as shown in Figure l. During with an annular pole piece or yoke plate 3 of this operation the pole piece it will slide a little similar material fastened to the open end of the one way or the other on top of the magnet l, field yoke l by means of machine screws 5 or the annular air gap at it being accurately main projection welding. Centrally located within the tained by means of the disc l5 which functions yoke I is apermanent magnet 'I of highretentivity 50 both as a centering disc and resilient clamping preferably Alnicoj composed of the following plate for holding the magnet elements in aselements, aluminum, nickel, cobalt, and iron. sembled operating position. This magnetic material is extremely hard, can- A voice coil il is wound upon a cylindrical form not be drilled or tapped, at least 'by ordinary it the top end of which is closed by a diaphragm means, and is moldedn'short lengths. The ua 55 2l. The voice coil structure is flexibly supported per and lower pole faces of the magnet l are for ibratory movement by means oi a concenground flat and klie in parallel planes. ln this trically corrugated centering disc of librous modiiication, the magnet Vl -is provided with a material supported at its periphery by a ring 25,

the latter being secured to the yoke t in any de- The base of the magnet Lrests upon a pole face so sired manner. it will be apparent to those skilled orground at surface of the bottom of the yoke in the art that the type of magnet 'l and hollow I the bore being `in alignment with a similar pole piece il, provided with central openings for the horn, cannot be bolted to the yoke I in the usual manner. This novel arrangement solves the diiculty.

Referring to the modication in Figure 2, the magnet 'l is centrally located upon the base of o Lhshape yoke as in Figures 4 and 5, and is preferably held in position as shown in these figures, described below. the top of magnet l and is provided with a laterally extending iiange 29. The yoke plate 3 is located upon the open end of yoke l in the same manner as shown in Figure 1. The pole piece 21 is centered within the opening I3 of plate 3 by means of a relatively stiff centering disc I6, preierably of brass or phosphor-bronze, fastened permanently to plate 3. A plurality of brass screws 3l are threaded in the disc l5 and extend up wardly 'through holes in plate 3 for turning to force the flange 29 downwardly against magnet 1, the clamping exerted axially by these screws 'and resiliency of the disc retaining the magneti-c elements in assembled relation. The assembly of parts easy in this case and requires no special tools. The parts may be taken apart readily. The centering disc and screws, in other words, are retaining means for both centering and clampingr the magnetic members.

Referring to Figure 3, the parts are assembled in Figure 2 with the exception that the nonmagnetic centering disc 33 is used solely to align the pole piece 35 and to accurately maintain an annular air gap. The screws 3l are threaded in the yoke plate 3, but extend through clearance holes in disc 33 to maintain a clamping action as in Figure 2. The pole piece 35 is provided with z;Y

a flange or shoulder of magnetically permeable material and preferably integral therewith, against which the screws exert pressure. A cupshape element 38, as of brass, is inverted over the flange 31, under the screw ends, the lip extending downward around the top of the magnet to limit lateral shifting of the latter. In this modication the magnet 3l and pole piece are of the usual solid type, used with a conventional irusto-conical type diaphragm 3S. During assembly of the magnet 34 laterally into position, the screws 3l, elements 3'! and 38 are raised suf- 'lciently to clear the top of the magnet.

deferring to Figure 4, I have illustrated the invention in connection with a magnet system similar to that shown in Gaubert et al. Patent 1,991,738 of February 19, 1935, and it may be considered in the nature of an improvement thereover. 1n the Gfaubert et al. patent the magnetic elements including a cobalt steel block magnet are held in assembled operating relation by means oi centering and clamping actions of a heavy nonmagnetic centering disc but with the aid or" a plurality of bolts which clamp the pole plate to the centering disc and the latter to the yoke. Such an arrangement, although a worth.- while improvement over the art, is somewhat complicated and expensive. In this modication the nagnet 53 is a small cylindrical bloclr of Alnico V an alloy of much higher magnetic energy than the older forms of Alnico, an alloy composed of aluminum 8%, nickel 14%, cobalt 241%, copper 3% and the rest iron. Because of tb characteristics of the material, and high magnetic energy storage in. a small lightweight piece, the present clamping system is ideally suitable. It been found undesirable to provide a bore hole in the magnet for bolting purposes. It had been the custom in making the heavier and larger magnets of ordinary Alnico alloy to provide a The pole piece 2'! is located on central bore hole at the time of molding, for bolting parts together, as shown in Cunningham Patent 2,312,238 of February 23, 1943, but such an arrangement had been found undesirable with the new alloy magnets.

In accordance with the present invention a yoke `plate 4l Figure 4, preferably rectangular, is welded to the ends of a U-shaped yoke 43. The bottom face oi plate 4l is provided with a milled groove concentric with opening I3, into which ts the upper edge of a dished circular centering element preferably of phosphor-bronze, the groove securing the centering element against lateral movement. This element is provided with a central. opening 41 accurately concentric with respect to opening i3 in the pole plate 4| and is normally biased downward in the same manner as shown in Figure 1. Element 45 is preferably slit radially as shown at 46 in Figure 4a, to provide a plurality of independently biased spring segments to provide greater spring action and more circumferentially uniform pressure around Eil. A pole piece 49, provided with a lower annular' flange l is assembled into approximately tl position shown by laterally inserting same relative to the yoke and upwardly into the po sition concentric with respect to and spaced from the opening i3 to form an air gap. Clamping too-,s are used during assembly to force the pole piece and spring element 45 upwardly slightly beyond the position shown and holding them in such position while the magnet 53 is inserted laterally into the position shown. The base of magnet preferably ts into a slight depression 54 formed by milling or pressing, to insure against lateral displacement in the case of severe shocks, as from dropping the unit. It will be apparent, therefore, that this modiiication results in a substantial simplification and improvement.

Referring to Figure 5, a disc 55 is used for centering the pole pieces and a separate spring clamp 5l' is used ior pressing the magnetic elements together, as shown and claimed broadly in combination in Cunningham application Serial No. 532,654, filed concurrently herewith. In this modification, however, each end of the spring clamp 5i' is biased upwardly and outwardly, as shown in dotted lines, and when assembled, engages an abutment or offset shoulder 59 with a high degree oi compression, holding the elements firmly together as shown. The assembly is accomplished simply by inserting the elements 49, 5l' and subsequently 53, with the spring tension relieved as the position shown in dotted lines. Then `figures of an arbor press are inserted to press the spring arms 51 downward until the ends snap into position against shoulder 59. In this modification, the assembly of the pole piece 49 and magnet 53 onto yolre 43 is substantially independent o the pole plate 4I and devices carried thereon., and ii desired the latter can subsequently be dropped into position with the centering disc fastened in place, and welded in the position shown. It is ordinarily advantageous to weld the pole plate in position irst but there are cases where the reverse is desired.

Another' feature of the Figure 5 modification is that several portions of the spring 51 are bent downwardly below flange 5l and annularly spaced closely around the top of the magnet 53 below the upper polar face for the purpose of limiting any lateral displacement of the magnet 53, as in Figure 3. This avoids the use of a depression (Fig. 4) or other means on the yoke around the base of the magnet, or the use of a separate element as in Figure 3. Ordinarily bolts, lugs, cement are used to anchor the magnet to prevent shifting, and in some cases the space between the magnet and yoke has been filled in with plastic molding material to hold all members together in a mass. With the present improvement, the portion of the eld yoke in contact with the magnet base may be ground to provide a smooth nat surface without any obstructions for ease of lateral assembly and providing an elcient contact area with the lower polar face of the magnet.

In Figure 6 the centering disc 60 is made of relatively stiif nonmagnetic material, is fastened permanently to plate M, as by welding, and the inner opening counterbored concentrically relative to yoke plate opening I3, nts snugly around inner pole piece 129. The resilient pressure effect is obtained in part or in entirety by separate means such as a soft rubber ring 58 heldin compression between the inner portion or the disc and the shoulder 5l on the base of the inner pole piece 49. Limitation of lateral movement of the magnet 53 is effected by lancing out several tongues 56, say four spaced ninety degrees apart, radially from the disc and bending them partway downward, as shown in dotted lines, in radially spaced relation. After assembling the magnet, the tongues are bent all the way down around the upper end of the magnet, as shown. The disc, therefore, serves for accurately centering the pole piece 49 concentrically with respect to the opening I3 in the plate 4I, for approximately centering the magnet, and cooperates with resilient means 58 for resiliently clamping the members 49 and 53 together and in place on the yoke, not shown, but similar in general to that of Figure 5. Similar lateral limiting structure may be formed in disc l5 oi Figures l, 2 or 4. Alternatively, rubber ring 58 may be discarded, and the centering disc engage the shoulder 5l, relying on the resiliency of the disc 60 to Clamp the magnetic members axially, as in Figure 1. In Figure 6 the disc, with or without the rubber ring, may be termed resilient centering means having structure for limiting lateral displacement of said magnet.

While the embodiments of the present invention as herein disclosed constitute preferred forms, it is to be understood that other forms might be adopted within the scope of the present invention and Within the scope of the claims which follow. While the invention has been illustrated in connection with a permanent magnet speaker, it will be found to have patentable utility with a field winding speaker.

I claim as my invention:

1. A dynamic loudspeaker comprising in combination (1) a field structure including a field yoke and a yoke plate connected to said eld yoke, said yoke plate having a centrally disposed circular opening, (2) a magnet having upper and lower pole faces With said lower pole face disposed on said field yoke, (3) a pole piece having one end thereof disposed upon said upper pole face, said pole piece having its opposite end extending into said opening in spaced relation to said yoke plate thereby to provide an air gap therebetween, and (4) an annular, disc-like spring secured to said yoke plate, said spring embracing and engaging said pole piece to center it in said yoke plate opening, said spring being biased by engagement thereof with said pole piece in a direction such as to cause it to exert a force against said pole piece in an axial direction toward said magnet whereby said spring is effective to maintain said pole piece,

8 said magnet and said iield structure in assembled relation.

2. The invention set forth in claim 1 wherein said pole piece has an annular shoulder against which said spring exerts its force for securely holding said pole piece, said magnet and said field structure in assembled relation.

3. In a dynamic loudspeaker, a lJ-shaped eld yoke of magnetic material having a base portion including a pole face intermediate the ends of said yoke, a yoke plate attached to said ends and provided with a circular opening, a permanent magnet having one end disposed upon said field yoke pole face, a pole piece having a lower pole face disposed upon the opposite end of said magnet and provided with an annular shoulder, spring means attached to said yoke plate and provided with an opening concentrically located with respect to said yoke plate opening, said spring means being of non-magnetic material and surrounding and engaging said pole piece for centering said pole piece within said yoke plate opening, said spring means being in contact with said annular shoulder and being biased by engagement thereof with said shoulder in a direction such as to Cause it to exert a force against said shoulder in an axial direction toward said magnet whereby said spring means is effective to securely hold said pole piece, said magnet and said yoke in assembled relation.

4. A dynamic loudspeaker comprising, in combination, (l) a field structure including a iield yoke and a yoke plate connected to said field yoke, said yoke plate having a centrally disposed circular opening, (2) a magnet having upper and lower pole faces with said lower pole face disposed on said field yoke, (3) a pole piece having one end thereof disposed upon said upper pole face, said pole piece having its opposite end extending into said opening in spaced relation to said yoke plate thereby to provide an air gap therebetween, and (-4) a sheet metal centering spring secured to said yoke plate, said spring embracing and engaging said pole piece to center it in said yoke plate opening, said spring being biased by engagement thereof with said pole piece in a direction such as to cause it to exert a force against said pole piece in an axial direction toward said magnet whereby said spring is eifective to maintain said pole piece, said magnet and said iield structure in assembled relation.

FRED J. HARBAUGH.

REFERENCES CITED The following references are of record in the file of this patent:

'UNITED STATES PATENTS Number Name Date 1,811,367 Pridham June 23, 1931 1,847,702 Thuras Mar. 1, 1932 1,864,809 Dempster et al. June 28, 1932 1,991,738 Gaubert et al. Feb. 19, 1935 2,029,232 Serge Jan. 28, 1936 2,312,238 Cunningham Feb. 23, 1943 2,395,166 Collins Feb. 19, 1946 FOREIGN PATENTS Number Country Date 372,809 Great Britain May 4, 1932 389,898 Great Britain Mar. 30, 1933 397,617 Great Britain Aug. 31, 1933 412,552 Great Britain June 21, 1934 552,328 Great Britain Apr. 1, 1943 827,276 France Apr. 22, 1938 

